Multi-dimensional optical and IR spectroscopy is a valuable tool for studying vibrational and electronic couplings and spectral correlations in multi-level systems.
Time-resolved, infrared (IR) spectroscopy is an important tool in physical chemistry and condensed matter physics. It is also of use in trace gas detection and atmospheric chemistry. In particular, mid-IR vibrational resonances provide a method for probing of molecular structure that can be combined with time-resolved techniques to extract information on molecular dynamics.
Owing to interest in characterizing spectroscopic transients at multiple vibrational frequencies in femtosecond IR experiments, there has been a need for multichannel IR array detectors. However, these arrays are presently limited by their finite size (typically 32 to 128 elements in linear arrays), and by their high cost. Unlike CCD arrays, traditional IR arrays are also very sensitive to thermal blackbody radiation, making thermal isolation a prerequisite for imaging. Most mid-infrared detectors must be liquid nitrogen cooled to obtain adequate sensitivity.
One of the current constraints of multi-dimensional spectroscopy is the experimental complexity of data acquisition, involving complicated pulse sequences and/or expensive spectral filters.
Multi-dimensional spectroscopy measures the total emitted field from a non-linear optical interaction in a quantum system. The complete measurement of the induced polarization has often been constructed using complex pulse sequences with multi-dimensional Fourier transform spectroscopy.
Pulsed Fourier transform spectroscopy typically uses three broadband pump pulses (focused in a non-colinear geometry) to generate a signal field in the phase-matched direction. The emitted ‘echo’ pulse is typically heterodyned with a fourth pulse and detected as a function of pulse separation. A multi-dimensional Fourier transform of the heterodyned field results in a frequency-frequency correlation spectrum of the transitions of interest. An alternative approach is double-resonance spectroscopy in which narrow-frequency-bandwidth excitation and detection fields are scanned.
There continues to be a need for further improvements in infrared spectroscopy to more efficiently measure structures and/or dynamics of interest.